An electrolytic capacitor is composed by using an anode electrode of the so-called valve metal such as aluminum or tantalum, said metal being caused to form an insulating oxide layer on its surface, said insulating oxide layer acting as a dielectric layer, contacting said insulating oxide layer with an electrolyte layer, arranging a current collector electrode generally referred to as cathode.
The electrolyte for the electrolytic capacitor, as above mentioned, contacts directly with the dielectric layer and acts as a true cathode, that is, the electrolyte lies between the dielectric layer of the electrolytic capacitor and the current collector electrode and hence its resistance is inserted in series for the electrolytic capacitor. Therefore, the characteristic of the electrolyte results in an important factor controlling the characteristic of the electrolytic capacitor. For example, if a conductivity of an electrolyte is lower, an equivalent series resistance within an electrolytic capacitor is increased and hence a high-frequency characteristic and a loss characteristic may be lowered.
That is the reason of the demand for high conductivity electrolyte and as such a high conductivity electrolyte, a solution of an inorganic acid, an organic acid or their salts in a proton accepting solvent, such as glycols or alcohols have been known. Especially, organic carboxylic acids have been used as salt of ammonium or primary, secondary or tertiary amine.
The latest expansion in demand for electrolytic capacitors having higher electric characteristics has not been satisfied with the conductivity of the presently available electrolyte. Especially in the case of the presently available electrolyte, when a desired conductivity is not obtained or a low solubility solute is used, the conductivity has been improved by intentially adding water.
However, in the latest utilizing condition of the electrolytic capacitor required for use at above 100.degree. C. and for many hours, the presence of water causes a dielectric layer to be worsened, an internal vapor pressure of the electrolytic capacitor to be increased, and hence, the life of the electrolytic capacitor is worsened because of a breakage of a seal part or a volatilization of an electrolyte. Therefore, the presently available electrolytic capacitor has had as a drawback the inability to hold a stable characteristic over a long term. To solve the matter, it is known that ammonium formate is used as a solute as disclosed in the Japanese patent publication No. 54-1023; ammonium propionate is used as a solute as disclosed in the Japanese patent publication No. 53-138047. However, when organic monocarboxylic acid salts of ammonium, primary or secondary amine are dissolved in a proton accepting solvent, an esterification or an amidification is caused at high temperature and such an ester or an amine does not remain in an ionic state and does not contribute to a conductivity and hence a high conductivity is not obtained. Further, even if an aprotic solvent is used, the same deterioration is caused by an amidification, and hence, a necessary characteristic of an electrolytic capacitor has not been obtained. Furthermore, when an organic monocarboxylic acid salt of a tertiary amine is used, such as the case of triethyl amine formate (the Japanese patent publication No. 52-45905), a deterioration due to an esterification is also caused in a proton accepting solvent. In this case, an esterification or amidification as mentioned above is not caused in an aprotic solvent. However, as to the conductivity, the requirement for the latest electrolytic capacitor has not been met.
It has been discovered that an alkyl ammonium salt of an aliphatic monocarboxylic acid has no such deterioration in an aprotic solvent as mentioned above, and the solution has an extremely high conductivity. Further, the alkyl ammonium salt of an aliphatic monocarboxylic acid may be used for a mixed solvent of an aprotic solvent and a proton accepting solvent such as ethylene glycol almost without the deterioration of the electrolytic capacitor during the characteristic life time. This may occur because the conventional ammonium salt or primary, secondary or tertiary amine salt dehydrates a proton accepting solvent. However, the alkyl ammonium salt of the invention removes alcohol, and the rate of the dealcohol reaction is extremely slow as compared with the rate of the dehydration reaction.